In general, kinds of liquid crystals are divided into a rod-type liquid crystal a coin-type discotic liquid crystal according to their shapes, and a material having at least 2 different indexes of 3-D refractive indexes nx, ny and nz is referred to as a birefringence material, and a direction showing no phase-difference generation of the light linearly polarized in an incident direction is defined as an optic axis, ad therefore a long-axis direction of a molecule becomes an optic axis in the rod-type liquid crystal, and a short-axis direction of a molecule becomes an optic axis in the discotic liquid crystal.
Among theses, the rod-type liquid crystal may be divided into five kinds of the alignment, as follows.
Firstly, a planar alignment means an alignment in which an optic axis is parallel to a plane of the film, secondly, a homeotropic alignment means an alignment in which an optic axis is vertical to a plane of the film, namely parallel to a normal of the film, and thirdly, a tilted alignment means an alignment in which an optic axis is declined at a certain angle between 0 to 90° to a plane of the film.
And fourthly, a splay alignment means an alignment in which an optic axis is continuously varied from 0° to 90°, or from the minimum value to the maximum value in the range of 0° to 90°, and fifthly, a cholesteric alignment means an alignment in which an optic axis rotates at a constant angle in a clockwise or a counterclockwise direction when it is viewed in a vertical direction to its plane as it proceeds toward a longitudinal direction while the optic axis is parallel to a plane of the film.
In the second homeotropic alignment amongst theses, a vertical alignment layer may be used for optical films such as a retardation film, a viewing-angle compensation film, etc. in liquid crystal display (LCD) devices such as a TN (Twist Nematic) mode, an STN (Super Twist Nematic) mode, an IPS (In Plane Switching) mode, a VA (Vertical Alignment) mode, an OCB (optically Compensated Birefringence) mode, etc. by using the vertical alignment layer alone or in combination with other film, and was prepared by coating a conventional alignment material to form a thin alignment layer, followed by coating a liquid crystal.
A glass has been mainly used for a substrate used to obtain such a vertical alignment layer, and alignment materials used on the glass substrate are mainly divided into organic alignment materials and inorganic alignment materials according to their kinds.
An example of the organic alignment materials includes lecitin, trichlorosilane and trimethoxypropyl silane, hexadecyl trimethyl ammonium halides, alkyl carboxylato monochromium salts, etc., and a representative example of the inorganic alignment materials includes SiO2 or MgF2, etc.
As a method for preparing a vertical alignment layer using the inorganic alignment material, a method for depositing SiO2 or MgF2 on a surface of a glass substrate has been known.
A principle of forming such a vertical alignment layer is described in brief as an example, as follows. If a small amount (generally, less than 1%) of the said organic alignment material is dissolved in a volatile solvent, coated on a substrate, and then the solvent is evaporated, then only a thin-film alignment material layer is present on the substrate, and if a liquid crystal is coated on the layer, then the liquid crystal is vertically aligned because a polar end group of an alignment material is adhered to a surface of glass having polarity, a hydrophobic alkyl chain is arranged in a vertical direction to the surface of glass, and a homeotropic alignment of liquid crystal molecules is attained by an attraction between the alkyl chain and the liquid crystal molecules.
Because the process of forming such an alignment layer was carried out by a washing process of the glass substrate, a printing process of the alignment layer, and drying and baking process in a sequential order, and electro-optical characteristics of a liquid crystal cell mainly depend on formation of the alignment layer and its surface treatment, it is important to form an alignment layer having a uniform thickness of an entire substrate in the printing process of the alignment layer, and very important to conduct a uniform surface treatment in the rubbing process.
That is to say, the characteristics of the alignment layer formed to arrange liquid crystal molecules in a uniform direction are generally desired to form a uniform thin film having a thickness of less than 1,000° C. at 200° C. or less and has an excellent adhesion property to a surface of the substrate, while a high chemical stability should be high so that it can not react with a liquid crystal, no charge trap should be present in an electrical property, and specific resistance should be sufficiently high so that it can affect no operation of the liquid crystal cell.
If it is possible to obtain a homeotropic alignment liquid crystal film having a homeotropic alignment property although a liquid crystal layer is directly coated on the substrate without coating an alignment layer, then the various and troublesome characteristics required for the alignment layer, as described above, and therefore many limited conditions in the manufacturing process may be never considered, and therefore merits such as simplicity and shortened manufacturing time of the process, and increased yield, etc. may be attained.
Korean Patent Application Publication No. 2002-0045547 disclosed a homeotropic alignment liquid crystal layer without such an additional alignment layer, wherein the used liquid crystal is a liquid crystalline polymer having a monomer unit including a liquid crystalline fragment side chain, and a monomer unit including a non-liquid crystalline fragment side chain. Such a liquid crystal polymer has a disadvantage that it is coated at a liquid state, and dried to remove a solvent, and then a high-temperature heat treatment process in a range of 70 to 200° C. is additionally required due to its higher glass transition temperature Tg than that of a reactive liquid crystal monomer, and it is impossible to apply to a rapid continuous process due to its extended time of 20 seconds to 30 minutes.
Meanwhile, if a glass substrate is used for manufacturing such a homeotropic alignment liquid crystal layer, then it is useful to homeotropically align a liquid crystal in LCD cells composed of glass, but the glass substrate is not suitable to obtain a homeotropic alignment liquid crystal layer so that it can adhere to a polarizing plate for the purpose of improvement of brightness, or viewing-angle compensation, etc.
That is to say, if a homeotropic alignment liquid crystal film is made of a glass substrate, then a roll-to-roll process, pressured while passing between rollers that face each other with a constant distance, should be carried out as described in the process of preparing the polarizing plate so as to adhere to a polarizing plate for the purpose of improvement of brightness, or viewing-angle compensation, etc., but it is nearly impossible to control the process so that the glass substrate can rapidly pass between the rollers without any damage to the glass substrate, and therefore it is difficult to mass-produce it due to such a problem.
In order to solve the problems, it may be desirable to use a plastic substrate having flexibility to pressure and slight impact other than a glass substrate as the substrate.
However, a plastic substrate may not be used as the aforementioned alignment material to obtain a vertical alignment layer since its surface does not have polarity as high as the glass surface.
Accordingly, some methods have been proposed so as to solve the problem shown if the plastic substrate is used instead of the glass substrate.
GB2324382A discloses a method wherein a material, in which a surfactant is fixed by a polymerizable liquid crystal material or a polymerizable high molecule on a plastic substrate, is used for an alignment layer.
Also, GB2358481A discloses a method wherein an aluminum film deposited on a plastic substrate is used for a vertical alignment layer.
However, both patent publications disclose a method wherein an alignment layer is firstly coated on a plastic substrate, and then a liquid crystal layer is coated, and if the deposited aluminum layer is particularly used as the alignment layer, then aluminum may be caused by defects of the plastic substrate since the aluminum adheres to its surface, and then some of the aluminum is removed off upon its peeling.
Accordingly, if a homeotropic alignment liquid crystal film may be prepared by directly coating a liquid crystal layer on a plastic substrate without coating an alignment layer, then it has excellent effects that it shows the simplicity of the process and the shortened processing time due to the omitted step of forming an alignment layer, as well as it may be mass-produced since the roll-to-roll process, which may not be obtained if a glass substrate is used, is possible, as described above.